1. Field of the Invention
The present invention relates to a light emitting device and a fabrication method thereof. More particularly, the present invention relates to a method of fabricating a substrate for a light emitting diode, on which patterns are formed using an anodic aluminum oxide (AAO) layer as a shadow mask; a vertical light emitting diode having a patterned semiconductor layer using an AAO layer and a fabrication method thereof, and a light emitting device having scattering centers using an AAO layer and a fabrication method thereof.
2. Discussion of the Background
A light emitting diode (LED), which is a representative one of light emitting devices, is a photoelectric conversion device having a structure, in which an N-type semiconductor and a P-type semiconductor are joined together, and emits predetermined light through recombination of electrons and holes. A GaN-based LED is known as such an LED. The GaN-based LED is fabricated by sequentially laminating a GaN-based N-type semiconductor layer, an active layer (or light emitting layer) and a P-type semiconductor layer on a substrate made of a material such as sapphire or SiC.
Light generated in an LED is not entirely emitted to the outside, but a large amount of light is lost inside the LED. Therefore, in order to enhance light efficiency of an LED, it is required to allow light generated from the LED not to be lost inside a semiconductor but to be emitted to the outside as much as possible.
When light passes through an interface between two media having different refraction indices, reflection and transmission of the light occur at the interface between the two media. If an incident angle is greater than a certain angle, transmission does not occur but total reflection occurs. In this case, the certain angle is referred to as a critical angle.
Due to the total refraction, when light emitted from an active layer proceeds toward a transparent electrode with an angle over the critical angle in an LED, the light is totally reflected at the transparent electrode and confined within the LED to be absorbed into an epitaxial layer and a sapphire substrate of the LED. Therefore, there is a problem that light efficiency of the LED may be lowered.
To solve such a problem, there is a method using a patterned sapphire substrate (PSS).
FIG. 1 is a view illustrating a method of fabricating a patterned sapphire substrate according to a prior art.
Referring to FIG. 1, a bent pattern 3 is formed on a sapphire substrate 1. A light emitting cell of an LED is grown on the pattern 3 of the sapphire substrate 1.
That is, before a semiconductor layer for forming a light emitting cell is grown, a bent pattern 3 with a specific shape is formed by patterning a sapphire substrate 1. Then, the semiconductor layer is grown on the bent pattern 3, thereby extracting the light that is not extracted to the outside of an LED due to total refraction.
As such, the inside light can be extracted to the outside by designing a structure of an LED to have a difference of refraction indices in a lateral direction.
However, in the prior art, since a photoresist layer 2 for forming the pattern 3 is formed on the substrate 1 and a pattern mask layer is then fabricated by photolithography for removing a predetermined region of the mask layer 2, the pattern mask layer is restricted by the size of the pattern. As the size of a pattern formed on a substrate is increased, a semiconductor layer should be grown to have a thickness greater than is necessary when the semiconductor layer is formed later.
In addition, as delicate photolithography is used, a fabrication process is difficult, fabrication cost is high, mass-productivity and reproductivity are lowered, and it is difficult to fabricate various shapes of patterns.
Meanwhile, since a nitride of a Group III element, such as GaN or AlN, generally has an excellent thermal stability and a direct transition type energy band structure, it has recently come into the spotlight as a substance for light emitting devices in blue and ultraviolet regions. Particularly, blue and green light emitting devices using GaN have been used in a variety of fields such as large-scale, full-color flat panel displays, traffic lights, indoor illumination, high-density light sources, high-resolution output systems and optical communications.
Such a nitride semiconductor layer of a Group III element, particularly GaN, is grown on a different kind of substrate with a similar crystal structure through a process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) because it is difficult to fabricate the same kind of substrate on which the nitride semiconductor layer can be grown. A sapphire substrate with a hexagonal system structure is mainly used as the different type of substrate. However, since sapphire is electrically nonconductive, the sapphire restricts a structure of an LED. Since the sapphire is mechanically and chemically stable, the sapphire is difficult to be subjected to processing such as cutting or shaping and has low thermal conductivity. Therefore, studies have been recently conducted to fabricate an LED with a vertical structure by growing nitride semiconductor layers on a different kind of substrate such as sapphire and then separating the different kind of substrate from the nitride semiconductor layers.
FIG. 2 is a sectional view of a vertical LED according to a prior art.
Referring to FIG. 2, the vertical LED has a conductive substrate 31.
The conductive substrate 31 may he a substrate made of Si, GaAs, GaP, AlGaInP, Ge, SiSe, GaN, AlInGaN, InGaN or the like, or a substrate made of a single metal of Al, Zn, Ag, W, Ti, Ni, Au, Mo, Pt, Pd, Cu, Cr or Fe, or an alloy thereof. Meanwhile, compound semiconductor layers are III-N-based compound semiconductor layers. First and second conductive types respectively designate N-type and P-type, or P-type and N-type.
Compound semiconductor layers including a first conductive semiconductor layer 15, an active layer 17 and a second conductive semiconductor layer 19 are formed on the conductive substrate 31. Further, an ohmic electrode layer 41, a metal reflective layer 43, a diffusion barrier layer 45 and a bonding metal layer 47 are interposed between the semiconductor layers and the conductive substrate 31.
The compound semiconductor layers are generally grown on a sacrificial substrate (not shown) such as a sapphire substrate by MOCVD or the like. Thereafter, the ohmic electrode layer 41, the metal reflective layer 43, the diffusion barrier layer 45 and the bonding metal layer 47 are formed on the compound semiconductor layers, and the conductive substrate 31 is then attached thereto. Subsequently, the sacrificial substrate is separated from the compound semiconductor layers using a laser lift-off technique or the like, and the first conductive semiconductor layer 15 is exposed. An electrode pad 33 is then formed on the exposed first conductive semiconductor layer 15. Accordingly, the conductive substrate 31 with excellent heat radiation performance is employed, thereby improving light emitting efficiency of an LED and providing the LED with a vertical structure as shown in FIG. 2.
In such a vertical LED, the first conductive semiconductor layer 15 may be generally divided into a low doped first conductive semiconductor layer 15a, which is grown at a low doping concentration when it is initially grown on a sacrificial substrate, and a high doped first conductive semiconductor layer 15b, which is grown at a high doping concentration on the low doped first conductive semiconductor layer 15a, depending on the doping concentration of dopant in growth of the first conductive semiconductor layer 15.
In order to improve an electrical characteristic and therefore a light emitting characteristic when the first conductive semiconductor layer 15 is exposed by the separation of the sacrificial substrate and the electrode pad 33 is then formed on the exposed first conductive semiconductor layer 15, the electrode pad 33 should he connected to the high doped first conductive semiconductor layer 15b rather than the low doped first conductive semiconductor layer 15a. 
To this end, after separating the sacrificial substrate, the low doped first conductive semiconductor layer 15a should be removed through wet or dry etching or back surface grinding to expose the high doped first conductive semiconductor layer 15b. However, in order to etch the low doped first conductive semiconductor layer 15a down to the high doped first conductive semiconductor layer 15b, it is important to manage the etching. Further, since the wet etching is defect etching, even the active layer 17 may be damaged. Furthermore, since a precise processing is required for the different surface etching, it is difficult to ensure evenness.